Switching mode power supplies are used in various applications. In order to get small volume and high efficiency, switching frequencies of switching mode power supplies are rather high, e.g., tens of thousands hertz. However, such high switching frequencies may cause severe electro-magnetic interference (“EMI”), which may decrease electrical quality, interfering electronic components around switching mode power supplies, interferes to radio wave, and/or may cause other negative impacts. As a result, strict EMI standards have been adopted in many countries.
In order to reduce EMI, one prior art technique involves distributing energy in a small switching frequency range to a large switching frequency range by varying or modulating the switching frequency within first and second frequency bands, or in other words, within a spectrum shaping range. However, the spectrum is fixed, which is not suitable for switching mode power supplies with variable switching frequencies. So there is a need to provide an improved switching mode power supply that can regulate a spectrum shaping range according to a switching frequency.